1. Field of the Invention
The present invention relates to a rotary head assembly used in a VTR, a R-DAT and the like. More specifically, the present invention relates to a rotary head assembly having a rotor of a motor for rotatably driving a rotary drum contained in the rotary drum.
2. Description of the Background Art
FIG. 11 is a cross sectional view of a conventional rotary head assembly. The rotary head assembly includes a rotary drum 1, a fixed drum 9, a motor 7 and a rotation shaft 13. The rotation shaft 13 is rotatably held by a ball bearing 15. An outer ring of ball bearing 15 is attached to fixed drum 9 by shrinkage fitting and adhesion.
A rotary drum 1 is arranged at a position opposing fixed drum 9. A disk 3 is attached to rotation shaft 13. Rotary drum 1 is attached to disk 3 by means of a fitting screw 2. If rotary drum 1 is directly attached to the shaft 13 of rotation, precision of height of a magnetic head 11 and the like becomes inferior. For this reason, rotary drum 1 is attached to disk 3 in order to improve precision in attaching rotary drum 1. Magnetic head 11 is attached on an end surface of rotary drum 1. Magnetic head 11 is rotated to scan a magnetic tape (not shown).
A rotor 5a of a rotary transformer is attached to disk 3 by adhesion. A stator 5b of the rotary transformer is attached to fixed drum 9 by adhesion. Rotor 5a and stator 5b of the rotary transformer are opposing to each other. A prescribed air gap is provided between rotor 5a and stator 5b of the rotary transformer. The rotary head assembly transmits and receives electric signals to and from magnetic head 11 by means of this rotary transformer. This rotary transformer is a flat type one.
Rotary drum 1 is arranged at one end portion of rotation shaft 13. Motor 7 is arranged at the other end portion of rotary shaft 13. Rotor 6 of the motor is attached to fixed drum 9. The shaft 13 is rotatably driven by means of motor 7. Members rotated by the rotation of shaft 13 include rotary drum 1, rotor 5a of the rotary transformer, disk 3 and magnetic head 11.
Problems of the rotary head assembly shown in FIG. 11 will be described in the following. Since the rotary head assembly has been reduced in size recently, rotation shaft 13 has been made thinner. If rotation shaft 13 is thin, it is likely to be twisted when rotation shaft 13 is rotatably driven by means of motor 7. When rotation shaft 13 is twisted, a jitter is generated. The jitter appears as a distortion on a TV screen.
A rotary head assembly disclosed in Japanese Patent Laying-Open No. 2-9013 has been proposed to solve this problem. FIG. 12 is a cross sectional view of the rotary head assembly disclosed in this Japanese Patent Laying-Open No. 2-9013. The rotary head assembly includes a rotary drum 17, a fixed drum 29 and a fixed shaft 31.
Fixed shaft 31 is inserted through ball bearing 41. A disk 43 is attached on an outer ring 27 of ball bearing 41. By press fitting the fixed shaft 31 to a hole portion 45 provided on the fixed drum 29, as shown in FIG. 13, the fixed shaft 31 is attached to fixed drum 29.
As shown in FIG. 12, rotary drum 17 is arranged to oppose to fixed drum 29. Rotary 17 is attached to disk 43 by means of a fitting screw 30. A head base 34 is attached on a side surface of rotary drum 17 by means of a fitting screw 36. A head chip 3 is attached at a tip end portion of head base 34. Head base 34 and head chip 35 constitute a magnetic head.
An earth ring 39 is attached to fixed shaft 31. Stator 25 of the motor is attached to earth ring 39 by means of a fitting screw 24. A yoke 23 is attached to rotary drum 17 by means of a fitting screw 26. A magnet 21 is attached to yoke 23. Yoke 23 and magnet 21 constitute a rotor of the motor. A rotary yoke 19 which is a metal plate is attached to rotary drum 17. By means of the rotary yoke 19, a magnetic path is formed between magnet 21 and rotary yoke 19. Therefore, magnetic force of magnet 21 is effectively applied to stator 25.
An earthing brush is attached to yoke 23. Earthing brush 37 is in contact with earth ring 39. When rotary drum 17 rotates, earthing brush 37 rotates around earth ring 39 while contacting earth ring 39. Rotary drum 17 is grounded by means of this earthing brush 37.
A stator 33a of rotary transformer is attached to fixed drum 29. A rotor 33b of rotary transformer is attached to rotary drum 17. Stator 33a and rotor 33b of the rotary transformer are opposing to each other. A prescribed air gap is provided between stator 33a and rotor 33b of rotary transformer.
In the rotary head assembly shown in FIG. 12, members which are rotated include rotary drum 17, rotary yoke 19, magnet 21, yoke 23, rotor 33b of the rotary transformer, head base 34, head chip 35, earthing brush 37, ball bearing 41 and disk 43.
In the rotary head assembly shown in FIG. 12, rotor (yoke 23, magnet 21) of the motor for rotatably driving rotary drum driving rotary drum 17 is attached to rotary drum 17. Therefore, even if the shaft 31 is thin, the fixed shaft 31 is not twisted.
The problems of the rotary head assembly shown in FIG. 12 will be described in the following.
(1) Processing of disk 43 is carried out by rotating disk 43. As the fixed shaft 31 is attached to fixed drum 29, processing of disk 43 becomes impossible. Therefore, before attaching fixed shaft 31 onto the fixed drum 29, disk 43 must be processed. In this case, if the fixed shaft 31 is attached inclined on the fixed drum 29, the rotary drum 17 cannot be attached but inclined.
(2) In the rotary head assembly, a flat type rotary transformer is used. Since the rotary head assembly is made smaller and smaller, the rotary transformer becomes thinner. The rotary transformer is processed by grinding. If the rotary transformer is thin, it may possibly warp during grinding. If such a rotary transformer is used, the air gap between the stator 33a and rotor 33b of the rotary transformer cannot be precisely provided.
(3) In the rotary head assembly, the rotary drum 17 is grounded by the contact between earthing brush 37 and earth ring 39 provided on the outer ring of fixed shaft 31. Resistance at the contact between earthing brush 37 and earth ring 39 has effected the precision of rotation of the rotary drum 17.
(4) Since the rotary head assembly is made smaller, thickness of the outer periphery of the rotary drum 17 is also made thinner. In order to rotate rotary drum 17 with less power consumption, it is preferred to position the motor on outermost side. Since the outer periphery of rotary drum 17 is thin and the motor is positioned on the outer side, magnetic tape is affected by the magnetic force of the motor.
(5) The fixed shaft 31 is attached to fixed drum 29 by press fitting. Therefore, it was difficult to accurately provide the air gap between rotor 33b and stator 33a of the rotary transformer.
(6) Rotor 33b of the rotary transformer to rotary drum 17 was done by adhesion. Therefore, rotor 33b of the rotary transformer could not be changed.